Handling emergency service calls made by subscribers poses several hurdles not encountered with, for example, land line devices. FIG. 1 illustrates an example of one of these hurdles. Consider the situation in which an emergency service call originates from a mobile station MS while moving in a high speed train. If the call is somehow disconnected or dropped before the emergency is completely reported, the Public Service Answering Point (PSAP) to which the emergency call was routed will attempt to call back the originator of the emergency call. However, as shown in FIG. 1, the caller may have moved from the coverage area 10 of an original wireless serving system to the coverage area 12 of a border system sometime after the call was dropped but before the call back could be completed. As a result, the mobile station might not be reachable by the border system to deliver the call back. A border system is one of a predetermined group of other wireless systems, the identity of which is determined by, for example, a network operator. Border systems do not necessarily have coverage areas bordering the original serving system.
Current solutions for emergency call back to a mobile station that has moved into a border system only work for when a mobile directory number (MDN) associated with the mobile station is known at the PSAP. The MDN of a mobile station is a dialable number. The MDN is dialed by a caller and used to route a call through the network to the wireless subscriber's home system. At the subscriber's home system, the home location register (HLR) contains the mobile subscriber identifier MSID associated with the subscriber's MDN. The MSID, not the MDN, is then used to route the call through the network to the serving wireless system and page the subscriber. The subscriber's MDN is provided by the home system to the serving system in a separate data file called the subscriber profile. Typically, the MSID is either a 10-digit mobile identification number (MIN) or a 15-digit International Mobile Subscriber Identifier (IMSI) programmed into a mobile station by the service provider with whom the mobile station user has entered into a service agreement. Accordingly, the MSID is not necessarily a dialable number.
In one solution for emergency call back where a mobile station has moved into a border system, the emergency call back is routed to the mobile station through the mobile station's home system, which then delivers the call to the border system according to existing standards. This process involves the use of temporary local directory numbers (TLDNs) signaled from the border system to the home system; wherein the TLDN assigned to the emergency call back is used by the home system to deliver the call to the border system.
As will be appreciated, this can be a time consuming process, particularly for an emergency call, and is especially acute if the mobile station is an international roamer or the home system and border system are separated by great distances.
Furthermore, as alluded to above, there is no solution for when the MDN of the emergency caller's mobile station is unknown. The MDN could be unknown for many reasons, including (a) the mobile station was never intended to be registered (there are such phones to use for emergency calls only), (b) the phone is new and has not yet been initialized by a service provider or (c) the subscription has expired and the mobile station is no longer registered with a service provider. Some mobile phones also support a removable User Identity Module (R-UIM) or Subscriber Identity Module (SIM) that may contain the MSID and the MDN. If the R-UIM or SIM are not in the mobile station, then the mobile station can still be used to place an emergency call. However, there is no MDN or MSID known to the mobile station or the serving system to provide the PSAP as a call back number.